1. Field of the Invention
The invention relates to a method of cooling a filtration apparatus immediately after the filtration apparatus is subjected to steam sterilization.
2. Description of the Related Art
A filtration apparatus is used for producing extrapure water useful as water for medicine, water for a hospital, water for electronic industries, or the like. Such a filtration apparatus is required to be periodically subjected to sterilization.
However, heat-resistant spores cannot be sufficiently sterilized by-hot water, so that steam sterilization is often required.
The steam sterilization is performed in the following manner. Steam (gage pressure: 1 kg/cm.sup.2, and 120.degree. C.) is continuously sent for a predetermined time period to the raw-water side of a filtration apparatus (the steam is continuously sent while condensed water is discharged from a steam trap). After the steam sterilization, a valve disposed immediately upstream from the steam trap is closed, and the filtration apparatus is isolated from the outside. Then, the residual steam in the apparatus is cooled. During the cooling, the residual steam is condensed and the internal pressure of the filtration apparatus becomes negative. This produces a fear that outside air may enter the apparatus and, because of the entry of the outside air, the interior of the filtration apparatus, or the filtrated-water side of a membrane module which is incorporated in the apparatus may be possibly contaminated. In order to prevent such contamination from occurring, the following methods have been proposed. A sterilized gas is introduced to the filtrated-water side immediately after the sending of steam is stopped, and the residual steam is cooled under the pressure of the introduced gas (Japanese Patent Publication No. Sho. 61-268303). Similarly, immediately after the stop of the sending of steam, hot water is introduced to the raw-water side and the residual steam is cooled together with the introduced hot water (Japanese Patent Publication No. Sho. 61-268303).
In the former method, however, it is necessary to provide sterilization equipment for the introduced gas and the introducing piping. In the latter method, it is necessary to provide hot-water producing equipment. In both of the methods, an increase of the equipment cost of a filtration apparatus, an increase of an installation space, and the like cannot be avoided. In the latter method, particularly, a huge amount of energy is consumed.
In order to eliminate the above-mentioned disadvantages of the cooling methods, the following method has been proposed (Japanese Patent Publication No. Sho. 63-190605). Immediately after the stop of the sending of steam, air is introduced from the raw-water side under pressure, and the filtrated-water side of the membrane module is filled with the air transmitted through the membrane (during the transmission through the membrane, sterilization is realized). Under the air-filled condition, cooling is attained to some extent, and residual steam is condensed. Because of the condensation, the pressure reduced. However, the pressure is prevented from becoming negative by means of the pressure of the air. In tilts way, the cooling is performed to some temperature. Then, raw water of an ordinary temperature is introduced into the filtration apparatus, and the final cooling is performed.
However, the method in which air is supplied under pressure and then raw water of an ordinary temperature is introduced involves the following drawbacks. Tale cooling is performed while a filtrated-water chamber of the membrane module is filled with the air. In addition, the filtrated water of the supplied raw water of an ordinary temperature is not caused to flow in the filtrated-water chamber. Therefore, the cooling rate on the filtrated-water chamber side of the membrane module is low and a condition in which the temperature of the filtrated-water chamber of the membrane module is largely different from that of the outer surface of the membrane module and that of the piping (a difference of about 20.degree. to 40.degree. C.) continues for a long time. It takes a long time to eliminate the difference (i.e., to perfectly cool the filtrated-water chamber side of the membrane module). When the membrane module starts to operate again before the finish of the cooling, and raw water is caused to flow under steady conditions, the membrane module may be easily damaged by a thermal stress.
For example, a membrane module may be configured so that a hollow fiber membrane flux is accommodated in a cylindrical cover, gaps between both inner ends of the cover and the hollow fiber membrane are sealed with an adhesive such as an epoxy resin, the interior of the hollow fiber membrane is used as a raw-water chamber, and the space between the hollow fiber membrane and the inner circumference of the cover is used as a filtrated-water chamber. In the membrane module, peeling due to a thermal stress easily occurs between the cover and the adhesive portion.